Ответ 1
Вы читали RFC?
См. раздел 4.1.2.1. Ошибка ответа для гранта авторизационного кода. В нем указаны коды ошибок, которые вы можете отправить обратно. server_error или temporarily_unavailable - это, вероятно, то, что вы ищете. Рекомендации по безопасности OAUth2 не вызывают причин не отправлять их обратно.
If the request fails due to a missing, invalid, or mismatching
redirection URI, or if the client identifier is missing or invalid,
the authorization server SHOULD inform the resource owner of the
error and MUST NOT automatically redirect the user-agent to the
invalid redirection URI.
If the resource owner denies the access request or if the request
fails for reasons other than a missing or invalid redirection URI,
the authorization server informs the client by adding the following
parameters to the query component of the redirection URI using the
"application/x-www-form-urlencoded" format, per Appendix B:
error
REQUIRED. A single ASCII [USASCII] error code from the
following:
invalid_request
The request is missing a required parameter, includes an
invalid parameter value, includes a parameter more than
once, or is otherwise malformed.
unauthorized_client
The client is not authorized to request an authorization
code using this method.
access_denied
The resource owner or authorization server denied the
request.
unsupported_response_type
The authorization server does not support obtaining an
authorization code using this method.
invalid_scope
The requested scope is invalid, unknown, or malformed.
server_error
The authorization server encountered an unexpected
condition that prevented it from fulfilling the request.
(This error code is needed because a 500 Internal Server
Error HTTP status code cannot be returned to the client
via an HTTP redirect.)
temporarily_unavailable
The authorization server is currently unable to handle
the request due to a temporary overloading or maintenance
of the server. (This error code is needed because a 503
Service Unavailable HTTP status code cannot be returned
to the client via an HTTP redirect.)
Values for the "error" parameter MUST NOT include characters
outside the set %x20-21 / %x23-5B / %x5D-7E.